wellknown: Convert Between 'WKT' and 'GeoJSON'

Convert 'WKT' to 'GeoJSON' and 'GeoJSON' to 'WKT'. Functions
included for converting between 'GeoJSON' to 'WKT', creating both
'GeoJSON' features, and non-features, creating 'WKT' from R objects
(e.g., lists, data.frames, vectors), and linting 'WKT'.